The secure and well sealed inter-engagement of the combustor and the high pressure turbine (HPT) stage immediately downstream therefrom is important in a gas turbine engine. This portion of the engine experiences the highest temperatures, and therefore the joint formed between the combustor and the HPT vane assembly must be able to accommodate thermal deflections of the two structures due to the high temperatures. Thermal growth mismatch between the combustor walls and the platforms of the HPT vane may occur for a variety of reasons, such as a result of differences in materials, structure, temperature variations, surface temperatures and gas flow patterns for example. Accordingly, the joints linking these two structures must be able to accommodate these deflections, without reducing sealing effectiveness. A tight seal therebetween during engine operation is required to prevent high pressure air, typically fed to the regions around the combustor, from leaking into the lower pressure gas flowing through the turbine section.
However, achieving an ideal clearance between the small exit duct (SED) and the HPT vane platform can be difficult, as too tight a clearance can cause binding between the two parts such that relative movement therebetween is limited and therefore thermal growth mismatch cannot be readily absorbed by a suitable joint arrangement, while too large a clearance gap therebetween can result in hot gas leakage and a reduction in the pressure differential required for impingement airflow cooling of the hot SED surfaces.